4.1 PL/SQL Performance in Oracle HTTP Server - Overview

This section describes several techniques to improve the performance of PL/SQL based Web applications in Oracle HTTP Server.

Table 4-1 lists recommendations for Database Access Descriptor (DAD) parameters and settings. By default, these DAD parameters are specified in the file dads.conf. On UNIX systems, this is in the ORACLE_HOME/Apache/modplsql/conf directory. On Windows systems, by default, this file is in the ORACLE_HOME\Apache\modplsql\conf directory. The file dads.README in this directory describes the DAD parameters in detail.

4.2 Process-Based and Thread-Based Operation in Oracle HTTP Server

This section describes PL/SQL performance issues that apply on platforms where the Oracle HTTP Server is process-based and thread-based. On a process-based Oracle HTTP Server, such as those running on UNIX-based platforms, each process servers all types of HTTP requests, including servlets and PL/SQL, static files. On a thread-based Oracle HTTP Server, such as Windows-based platforms, there is just one Oracle HTTP Server process with multiple threads within the process; individual threads can be used serve all types of HTTP requests.

Note:

In some cases in this chapter we make references to performance optimizations that apply for PL/SQL based Web applications where the distinction between platforms, either process-based or thread based is significant.

4.3 Performance Tuning Issues in mod_plsql

While using mod_plsql, the areas that affect performance and scalability are:

4.3.1 PL/SQL Based Web Application Development Considerations and Programming Tips

PL/SQL Gateway users should consider the following topics when developing PL/SQL based Web applications:

Manage the use of Database Access Descriptors (DADs)

Try to restrict the number of DADs that each Oracle HTTP Server node uses.

Note:

Performance is not affected if there are DADs that are not being used.

Use of Nested Tables

PL/SQL provides the ability to create tables. To build PL/SQL tables, you build a table that gives the datatype of the table, as well as the index of the table. The index of the table is the binary integer ranging from -2147483647 to +2147483647. This table index option is known as sparsity, and allows meaningful index numbers such as customer numbers, employee number, or other useful index keys. Use PL/SQL tables to process large amounts of data.

PL/SQL provides TABLE and VARRAY (variable size array) collection types. The TABLE collection type is called a nested table. Nested tables are unlimited in size and can be sparse, which means that elements within the nested table can be deleted using the DELETE procedure. Variable size arrays have a maximum size and maintain their order and subscript when stored in the database. Nested table data is stored in a system table that is associated with the nested table. Variable size arrays are suited for batch operations in which the application processes the data in batch array style. Nested tables make for efficient queries by storing the nested table in a storage table, where each element maps to a row in the storage table.

Use procedure naming overloading with caution

PL/SQL based Web applications should use the procedure name overloading feature with caution. It is best if procedure name overloading is avoided by having multiple procedures with different names.

Consider rewriting applications where there is significant overhead in determining the type parameters

PL/SQL based Web applications should be aware of the overhead in trying to execute procedures where the URL does not provide enough details to know about the type of the parameter, such as scalar or array. In such cases, the first attempt to execute a procedure fails and the procedure signature needs to be described before it can be executed by mod_plsql.

4.3.2 Connection Pooling Tips and Oracle HTTP Server Configuration

Consider the following topics when configuring connection pooling with Oracle HTTP Server:

Using the default connections pooling and setting values for PlsqlMaxRequestsPerSession

Creating new database connections is an expensive operation and it is best if every request does not have to open and close it own database connections. The optimal technique is to make sure that database connections opened in one request are reused in subsequent requests. In some rare situations, where a database is accessed very infrequently and performance is not a major concern, connection pooling can be disabled. For example, if the administrator accesses a site infrequently to perform some administration tasks, then the DAD used to access the administrator applications can choose to disable connection pooling. To disable connection pooling, set the DAD parameter PlsqlMaxRequestsPerSession to the value 1.

Note:

Setting PlsqlMaxRequestsPerSession to the value 1 reduces the number of available database sessions and may impact performance.

On UNIX systems, Oracle HTTP Server configuration should be properly tuned so that once processes are started up, the processes remain up for a while. Otherwise, the connection pooling in mod_plsql is rendered useless. The Oracle HTTP Server listener should not have to continually start up and shut down processes. A proper load analysis should be performed of the site to determine what the average load on the Web site. The Oracle HTTP Server configuration should be tuned such that the number of httpd processes can handle the average load on the system. In addition, the configuration parameter MaxClients in the httpd.conf file should be able to handle random load spikes as well.

On UNIX systems, Oracle HTTP Server processes should be configured so that processes are eventually killed and restarted. This is required to manage any possible memory leaks in various components accessed through the Oracle HTTP Server. This is specifically required in mod_plsql to ensure that any database session resource leaks do not cause a problem. Make sure that MaxRequestsPerChild configuration parameter is set to a high number. For PL/SQL based Web applications, this should not be set to 0.

For heavily loaded sites, the Oracle HTTP Server configuration parameter KeepAlive should be disabled. This ensures that each process is available to service requests from other clients as soon as a process is done with servicing the current request. For sites which are not heavily loaded, and where it is guaranteed that the number of Oracle HTTP Server processes are always greater than the number of simultaneous requests to the Oracle HTTP Server listener, enabling the KeepAlive parameter results in performance improvements. In such cases, make sure to tune the KeepAliveTimeout parameter appropriately.

You may want to lower the value of Timeout in the Oracle HTTP Server configuration. This ensures that Oracle HTTP Server processes are freed up earlier if a client is not responding in a timely manner. Do not set this value too low, otherwise slower responding clients could time out.

Most Web sites have many static image files, which are displayed in each screen for a consistent user interface. Such files rarely change and you can reduce a considerable load on the system by tagging each image served by the Oracle HTTP Server listener with mod_expires. You should also consider front-ending your Web site with Oracle Application Server Web Cache.

How do I know if the Web site can benefit from the use of mod_expires?

Use Netscape, or any browser that enables you to view page caching information, and visit several heavily accessed Web pages on the site. On each page, right click the mouse and select View Info from the pop up menu (or the equivalent command for your browser). If the top panel in the page information window lists many different images and static content, then the site could benefit from the use of mod_expires.

You can also check the Oracle HTTP Server access logs to see what percentage of requests result in HTTP 304 (Not Modified) status. Use the grep utility to search for 304 in the access_log and divide this resulting number of lines by the total number of lines in the access_log. If this percentage is high, then the site could benefit from the use of mod_expires.

How do I tag static files with the Expires header?

Locate the Location directive used to serve your static image files. Add the ExpiresActive and ExpiresDefault directives to it.

How do I know if the static files are being tagged with the Expires header?

Using Netscape, or the browser of your choice, clean up all the cached files in the browser.

Visit a Web page that should have images tagged with the Expires header. Right click the mouse on the page and select View Info, from the pop up menu. or use the equivalent command for your browser.

In the top panel of the page information, select an image that should be tagged with the Expires header.

Review the information displayed in the bottom panel. The Expires header should be set to a valid date. If this entry is No date given, then the file is not being tagged with the Expires header.

4.3.3 Tuning the Number of Database Sessions

Consider the following topics when tuning the number of database sessions:

The processes and sessions parameters in the Oracle init$SID.ora configuration file should be set so that Oracle is able to handle the maximum number of database sessions. This number should be proportional to the number of DADs times the maximum number of Oracle HTTP Server processes, times the number of Oracle HTTP Server instances.

On UNIX platforms, the connection pool is not shared across Oracle HTTP Server processes. For this reason, it is recommended that the application use as few DADs as possible.

Front ending your Oracle HTTP Server with Oracle Application Server Web Cache reduces the requirement to have a high number of processes for your HTTP configuration, resulting in lesser number of database sessions.

4.3.4 Two-Listener Strategy

On platforms where the Oracle HTTP Server is process-based, such as all UNIX-based platforms, each process serves all types of HTTP requests, including servlets, PLSQL, static files, and CGI. In a single Oracle HTTP Server listener setup, each httpd process maintains its own connection pool to the database. The maximum number of database sessions is governed by the setting in httpd.conf configuration file for StartServers, MinSpareServers, and MaxSpareServers, plus the load on the system. This architecture does not allow for tuning the number of database sessions based on the number of mod_plsql requests. To tune the number of database sessions based on the number of mod_plsql requests, install a separate HTTP listener for mod_plsql requests only. This approach greatly reduces the number of database sessions that are needed to serve mod_plsql requests.

For example, assume a main Oracle HTTP Server listener is running on port 7777 of mylsnr1.mycompany.com. First, you can install another Oracle HTTP Server listener on port 8888 on mylsnr2.mycompany.com. Next, redirect all mod_plsql requests made to mylsnr1.mycompany.com:7777 to the second listener on mylsnr2.mycompany.com:8888. Review the following steps:

To redirect all PL/SQL requests for mylsnr1.mycompany.com:7777 to mylsnr2.mycompany.com:8888, make the following configuration changes:

For the Oracle HTTP Server listener running on Port 7777, edit ORACLE_HOME/Apache/modplsql/conf/plsql.conf file. Comment out the following line by putting a # in front of the line:

#LoadModule plsql_module...

Copy the DAD location used to service PL/SQL requests in mylsnr1.mycompany.com to the configuration file ORACLE_HOME/Apache/modplsql/conf/dads.conf in mylsnr2.mycompany.com.

Comment out the DAD location configuration parameters on mylsnr1.mycompany.com by prefixing the line with a "#" character.

#<Location /pls/portal>
#...
#</Location>

Configure this listener to forward all mod_plsql requests for this DAD location to the second listener by adding the following line in dads.conf:

ProxyPass /pls/portal http://mylsnr2.mycompany.com:8888/pls/portal

Repeat the configuration procedures for all DAD Locations.

Because the PL/SQL procedures generate URLs that are displayed in the browser, it is important that all URLs are constructed without any references to the internal mod_plsql listener on mylsnr2.mycompany.com:8888. Depending on how the URLs are being generated in the PL/SQL based Web application, there are three options:

If the URLs are hard-coded into the application, make sure that they are always generated using the hard-coded values as HOST=mylsnr1.mycompany.com and PORT=7777. No change would be required for this scenario.

If the PL/SQL based Web applications always use the CGI environment variables SERVER_NAME and SERVER_PORT, then it is easy to change the configuration of the listener on mylsnr2.mycompany.com. Edit the file and change the lines ServerName and Port in the ORACLE_HOME/Apache/conf/httpd.conf file for the second listener as follows:

If the URLs are being generated using the CGI environment variable HTTP_HOST, you need to override the CGI environment variables for the Oracle HTTP Server listener running on Port 8888. Add the following lines to the ORACLE_HOME/Apache/modplsql/conf/dads.conf file for each DAD to override the default CGI environment variables HOST, SERVER_NAME, and SERVER_PORT:

In all cases, the intent is to fool the application to generate URLs as if there never was a second listener.

Test the setup and make sure that you can access all the DADs without any problems.

In this setup, the main listener mylsnr1.mycompany.com can be configured based on the total load on the Oracle HTTP Server listener. The second listener on mylsnr2.mycompany.com can be fine-tuned based on just the mod_plsql requests being made.

4.3.5 Overhead Problems

While executing some of the stored procedures, mod_plsql may incur a Describe overhead, which would result in two extra round trips to the database for a successful execution. This has performance implications.

4.3.5.1 The Describe Overhead

In order to execute PL/SQL procedures, mod_plsql needs to know about the datatype of the parameters being passed in. Based on this information, mod_plsql binds each parameter either as an array or as a scalar. One way to know the procedure signature is to describe the procedure before executing it. However, this approach is not efficient because every procedure has to be described before execution. To avoid the describe overhead, mod_plsql looks at the number of parameters passed for each parameter name. It uses this information to assume the datatype of each variable. The logic is simply that if there is a single value being passed, then the parameter is a scalar, otherwise it is an array. This works for most cases but fails if there is an attempt to pass a single value for an array parameter or pass multiple values for a scalar. In such cases, the first attempt to execute the PL/SQL procedure fails. mod_plsql issues a Describe call to get the signature of the PL/SQL procedure and binds each parameter based on the information retrieved from the Describe operation. The procedure is re-executed and results are sent back.

This Describe call occurs transparently to the procedure, but internally mod_plsql has encountered two extra round trips, one for the failed execute call and the other for the describe call.

4.3.5.2 Avoiding the Describe Overhead

You can avoid performance problems with the following:

Use flexible parameter passing.

Always ensure that you pass multiple values for arrays. For single values, you can pass dummy values that are ignored by the procedure.

Use the following workaround, which defines a two-parameter style procedure which defaults the unused variables.

Define a scalar equivalent of your procedure, which internally calls the original procedure. For example, the original package could be similar to the following example:

4.3.6 The Flexible Parameter Passing (four-parameter) Overhead

Round-trip overhead exists if a PL/SQL procedure is using the older style four-parameter interface. The PL/SQL Gateway first tries to execute the procedure by using the two-parameter interface. If this fails, the PL/SQL Gateway tries the four-parameter interface. This implies that all four-parameter interface procedures experience one extra round-trip for execution.

Avoiding the flexible parameter passing overhead

To avoid this overhead, it is recommended that you write corresponding wrappers that use the two-parameter interface and internally call the four-parameter interface procedures. Another option is to change the specification of the original procedure to default to the parameters that are not passed in the two-parameter interface. The four-parameter interface has been provided only for backward compatibility and will be deprecated in the future.

Using flexible parameters and the exclamation mark

The flexible parameter passing mode in Oracle HTTP Server expects the PL/SQL procedure to have the exclamation mark before the procedure name. Due to performance implications of the auto-detect method used in Oracle HTTP Server, the exclamation mark is now required for flexible parameter passing in Oracle HTTP Server. In Oracle HTTP Server, each procedure is described completely before being executed. The procedure Describe call determines the signature of the procedure and requires around-trip to the database. The PL/SQL Gateway in Oracle HTTP Server avoids this round trip by having end-users explicitly indicate the flexible parameter passing convention by adding the exclamation mark before the procedure.

4.4 Tuning File System Cache to Improve Caching Performance

You can configure and use a File System Cache to improve the performance of OracleAS Portal applications and generic PL/SQL based Web applications.

4.4.1 Introducing File System Cache Tuning

This section covers mod_plsql related File System Cache tuning options. Cache contents are cached using Operating System supplied file system calls; the cached contents are not stored in the mod_plsql memory space. Using the mod_plsql File System Cache, the contents of the cache may be in memory when the Operating System supports, and the system is configured to use features such as memory disk (some UNIX platforms support memory disk based fast storage).

The information in this section can improve the performance of PL/SQL based Web applications when mod_plsql is configured to use the File System Cache. For example, OracleAS Portal uses the File System Cache, and therefore, OracleAS Portal performance should improve when the File System Cache is properly tuned.

Table 4-3 lists the cache related parameters that you can set for mod_plsql. Set these parameters in the cache.conf file that is available on UNIX in the directory, ORACLE_HOME/Apache/modplsql/conf, and on Windows, this is found in the directory, ORACLE_HOME\Apache\modplsql\conf.

Note:

The file cache.README in the conf directory includes a full description of each parameter, and provides examples showing how to set parameter values.

4.4.2 Enabling File System Cache

The cache.conf parameter PlsqlCacheEnable enables mod_plsql caching. For maximum performance, enable PlsqlCacheEnable by setting the value of this parameter to On.

Note:

Only applications that support PLSQL caching, such as Oracle Portal, will benefit by setting PlsqlCacheEnable to On.

4.4.3 Configuring File System Cache to Reside on a Faster File System

This section describes how to configure a File System Cache to reside on a separate disk. When you use File System Cache and store the cache on a faster separate disk, performance should improve for all types of Web applications using File System Cache, including OracleAS Portal and generic PL/SQL based Web applications.

When you configure File System Cache, the cache can reside either on a separate physical disk or in a memory disk.

To set up a File System Cache on a separate disk:

Assume that the file system for the cache resides at the location:

On UNIX: /u01/cache

On Windows: E:\cache

Update the file:

On UNIX: ORACLE_HOME/Apache/modplsql/conf/cache.conf

On Windows: ORACLE_HOME\Apache\modplsql\conf\cache.conf

Change the cache parameter PlsqlCacheDirectory:

On UNIX: PlsqlCacheDirectory /u01/cache

On Windows: PlsqlCacheDirectory E:\cache

Restart Oracle HTTP Server for the configuration changes to take effect.

This section provides tips and techniques for reducing the time required to access and login to OracleAS Portal.

Note:

The information in this section only applies only to OracleAS Portal, and only when OracleAS Portal is configured in the mid-tier.

The information in this section only applies on UNIX systems.

Given that each OracleAS Portal session cache item is small, accessed for each request, and is volatile, you can improve the session cache performance by ensuring that the session cache resides on a performant file system.

Options for a more performant file system include memory-based file systems that are available on many UNIX platforms.

OracleAS Portal uses session cookies to maintain session details for each OracleAS Portal user. The session cookie is encrypted and contains important information, including: the database username, the lightweight username, and the Globalization Support characteristics of the session. In order for mod_plsql to execute a OracleAS Portal request, it needs to get hold of the database username from the session cookie. To avoid performing an expensive decrypt operation with each user request, mod_plsql decrypts the session cookie once and maintains the relevant details in a OracleAS Portal session cache that is stored on the local file system.

This calculation only applies for session cookies that get created in one day. The total OracleAS Portal session cache calculation does not account for session cache items that got created earlier on.

There is no configuration parameter to specify how much space to use for the OracleAS Portal session cache. The session caching logic will keep writing to the session cache directory and assume that the session cache cleanup is done frequently enough to make sure that the disk never runs out of space. If you are using your swap space for the session cache, it is extremely important that you never run out of swap space on the system. If the disk does run out of space, session caching will not occur, but OracleAS Portal will continue to run with a substantial degradation in performance. This will adversely affect the system in general and should be monitored

Identify a file system with enough space to hold the OracleAS Portal session cache; then, create a session cache directory.

If you want to use the swap space in /tmp for the OracleAS Portal session cache, you need to do the following

Login as the user that will be running the Apache listener and the OC4J instances.

Create a new directory for the OracleAS Portal session cache. For example, using the command:

% mkdir /tmp/session

Revoke all privileges for everyone else on the new directory:

% chmod 700 /tmp/session

Caution:

No other component in the system should be creating files under the OracleAS Portal session cache directory. Make sure that you create a new directory for the session cache.

4.4.5.1 Setting the Total Cache Size with PlsqlCacheTotalSize

The default installation sets the mod_plsql file system cache size to 2097152 bytes (20 Megabytes). If your PL/SQL application does not make use of the OWA_CACHE packages, or uses them to cache small amounts of content, then the default setting should be sufficient. If your PL/SQL application caches a lot of content in the mod_plsql file system cache, you should consider specifying a higher value.

To control the cache size, set the PlsqlCacheTotalSize parameter in the file cache.conf. On UNIX systems, this file is located under ORACLE_HOME/Apache/modplsql/conf directory. On Windows systems, this file is located under ORACLE_HOME\Apache\modplsql\conf.

You need to set the cache size high enough to achieve a high cache hit ratio. Try to set the cache size large enough so that frequently accessed content stays cached. It is also important to limit the amount of disk space, so that the cache size does not grow too large. Correct tuning for the cache size provides enough cache to hold all frequently accessed content while preventing the cache size from growing too large, since a very large cache is inefficient to search.

The value for PlsqlCacheTotalSize is specified as a number of bytes. 1MB equals 1048576 bytes. This setting is a soft limit on the amount of cache allocated. In some cases, the cache size may grow beyond this limit until the next cleanup operation. Therefore, the hard limit on the cache size is the underlying physical hard disk size. When this limit is reached, no cache content can be written out to disk until space is available.

Turn on mod_plsql performance logging by setting the LogLevel in httpd.conf to the info level to enable mod_plsql logging.

Monitor the error_log on a daily basis. On UNIX systems, the default directory for the error log is: ORACLE_HOME/Apache/Apache/log. On Windows systems, the default directory is: ORACLE_HOME\Apache\Apache\log.

deleted is the number of cache files that got deleted during the cleanup process.

max_age is the number of cache files and total size that got deleted because they haven't been used for some time.

kept is the number of cache files and total size that was kept after the cleanup process.

time is the amount of time to perform the cleanup.

limit is the total cache size. This is the value of the PlsqlCacheTotalSize setting.

Interpret the entries in the error log as follows:

If a high number of files are being deleted when compared to the number of files that were kept, this is a clear indication that your cache size is too small. You probably need to increase the size of the cache.

If a low number of files being deleted when compared to the number of files that were kept is observed, this is an indication that your cache size is probably too big. If you have enough disk space, you can chose to leave it as it or you can decrease the size of the cache to reclaim some disk space.

4.4.5.2 Setting the Days of Aging for Cache with PlsqlCacheMaxAge

Using the PlsqlCacheMaxAge parameter, you can control the "staleness" of cache content. The value for parameter is specified in units of days. The default value for this parameter is 30 (days). This means cache content is kept in the cache if it is less than 30 days old. After 30 days, the content is considered for deletion during the cleanup process.

The max_age information in mod_plsql error_log shows cache file aging information. If your site is a highly dynamic site, it would make sense to configure this setting to a lower value, since the older cache content will usually not be used again and, therefore, the lower value does not affect the cache hit ratio. If the site contains many static pages, it would make sense to increase the value of PlsqlCacheMaxAge so that the cleanup process does not deliberately delete the cache content.

4.4.5.3 Setting the Maximum File Size for a Cache File with PlsqlCacheMaxSize

Using the PlsqlCacheMaxSize parameter, you can specify the maximum size for individual files in the cache. Using this parameter prevents the case in which one cache file fills up the entire cache.

The default value for this parameter is 1048576 (bytes). In general, set this parameter to a value that represents about 1-3% of the total cache size.

4.4.6 Configuring Cache Cleanup

The cache cleanup parameter determines the frequency in which the File System Cache is examined and, if necessary, cleaned up. The cache cleanup parameter, PlsqlCacheCleanupTime is specified in the cache.conf file. The frequency can be set to daily, weekly, or monthly. When specifying weekly cleanup, it is possible to specify the day of the week and the time of the day.

The default mod_plsql setting of PlsqlCacheCleanupTime is daily at 11PM local time. Therefore, by default, every night at 11PM, the cleanup routine runs. When you select the monthly frequency, the cleanup occurs on the first Saturday of each month.

Configuring this parameter correctly is important since cleaning up too often can result in a lower cache hit ratio and when cleaning does not occur often enough, the cache's disk usage may be excessive.

Monitor the cleanup activities using the entries in the mod_plsql error_log; then tune the cleanup parameter, PlsqlCacheCleanupTime by analyzing the entries.

Seeing a large number for the cleanup time can be an indication that the cleanup frequency is set too low. When the log indicates that the cleanup operation is busy examining or deleting many cache files, increasing the cleanup frequency should decrease the time spent in the cleanup operation.

If a high number files are being deleted during the cleanup operation because of "staleness", this is an indication that the cleanup frequency is too low. In this case, increase the frequency so that the cleanup can actively delete "stale" cache content more frequently.